Thursday, 24 January 2013

The hazards posed and some causes of tropical storms.

 I spent a large proportion of time in vein, but in the end I managed to scrounge these little gems from two academic journals published in 1994 and in 1991. Some quite interesting stuff here:

Types and causes of tropical storms:
The largest and most common type of tropical storm is the temperate or mid-latitude cyclone that forms as a result of rising air motion induced by waves in the jet stream. These low pressure systems derive their energy from the strong horizontal temperature gradients and are called "cold core" cyclones by meteorologists. Mid-latitude cyclones are the familiar low pressure regions that produce the majority of the cool season precipitation. In most of these cyclones, the pressure gradient between the centre and the outer margins of the systems is modest, averaging about 5 mb/100 km. Consequently, the associated wind fields are relatively weak, ranging from 20 to 30 kmh
However, there are times when the inducing factors, such as the presence of a substantial temperature contrast as the cyclones cross the Gulf Stream, or a sharp difference in temperature between land and the ocean, can combine to produce very strong temperate cyclones off the mid Atlantic coast. Tropical cyclones and their mid-latitude counterparts bear little resemblance apart from their cyclonic winds. Tropical cyclones are warm-core systems, in that the air temperatures at their centers are higher than those in the surrounding regions.
These cyclones form over the open ocean and get their energy from the evaporation of warm ocean water. The low pressure centre of a hurricane is augmented and strengthened in part by a massive ring of thunderstorms which release enormous amounts of heat into the storm system. The most powerful hurricanes will commonly have a 20 mb/100 km pressure gradient, which results in more powerful wind field.
The strongest winds are usually concentrated over a much smaller area than those associated with a mid-latitude cyclone. Also rather interestingly, less than five percent of all tropical disturbances develop into hurricanes. Seems bizarre but understandable from the explanation.
Coastal Storm Hazards. Robert Dolan and Robert E. Davis. Journal of Coastal Research, Special Issue No. 12. COASTAL HAZARDS: PERCEPTION,SUSCEPTIBILITY AND MITIGATION (1994), pp. 105 Coastal Education & Research Foundation, Inc.

Hazards associated with tropical storms in the mid-Atlantic region specifically:
One topic simply wasn't enough for me; I couldn't contain myself and I stumbled across this which seems quite self-explanatory but it's not the sort of thing I think about when I'm day dreaming. But apparently it is not the wind fields associated with tropical and extratropical storms that represent the most serious hazard, but rather the waves and storm surges that the winds produce. Sooner or later, all waves produced by these storms end up dissipating most of their potential and kinetic energy in the coastal zone. The growth of waves in a storm from the initial ripples and wavelets is governed by three factors: the wind speed, its duration, and the distance (fetch) of water surface over which it blows. With an increase in any of these factors, the height of the waves and the potential storm surge in creases. Wave hindcasting methods and storm surge predictions were developed by extension of the basic physics of wave generation. Wave heights generated by hurricanes typically range from 5 m to 15 m
Temporal Variation of Tropical Cyclones in the North Atlantic Basin. Anthony J. Vega and Mark S. Binkley. GeoJournal, Vol. 23, No. 4, Caribbean Hurricanes (April 1991), pp. 313 Springer

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